Any cutting tool engaged in machining, continuous or interrupted, is subjected to wear. The development of wear lowers machining efficiency, leads to poor surface quality and dimensional errors on the component being machined and, if left unattended, could result in catastrophic tool breakage with dangerous consequences.
There are several characteristic tool wear modes and these are schematically shown in FIG. 1 and described below:
Flank Wear (FW). This is the typical, regular type of abrasive wear occurring on the clearance surface of the cutting tool, primarily due to abrasion between the tool and the machined surface. It occurs on the flank of the primary cutting edge and the nose of the tool. (The former is referred to as flank wear and the latter as nose wear for the purpose of distinction, in this report). PA1 Crater Wear (CW). Another regular type of wear occurring on the rake face of the cutting tool, usually due to inter-molecular diffusion between the tool and the chip. PA1 Chipping, Cracks, or Breakage of the Edge (CH, CR, BR). This is an irregular type of tool wear and associated with brittle fracture of the cutting edge. PA1 Plastic Deformation of the Tool Edge (PD). This is due to loss of compressive strength of the material at high temperatures generated during cutting and may contribute to both flank and crater wear. PA1 Notching (N). This is the excessive flank wear that occurs at end of the depth of engagement between tool and workpiece and is caused by the rubbing of the work hardened top layer produced by the previous cut on the workpiece. PA1 1. Evaluate overall tool condition during rough machining (flank wear of cutting edge and nose wear). PA1 2. Evaluate wear processes that affect workpiece dimensional accuracy during finish machining (nose wear, see FIG. 2). PA1 First Objective: To evaluate sensors and analyses of sensor outputs to establish the accuracy precision and applicability of different cutting tool monitoring techniques for controlling production metal cutting processes. PA1 Second Objective: To develop and demonstrate an RTTC monitor based on the best procedure established in the first year's research.
Of these various modes, proper choice of tool material-workpiece material combination can eliminate all but flank wear on the nose and the primary cutting edge of the tool. Moreover flank wear of the nose and the cutting edge and catastrophic breakage, due to a variety of reasons, are the only modes of wear that impact the quality of the workpiece and the efficiency of the machining process. Consequently only these modes of tool wear are significant and to be considered in the Real-Time Tool-Condition (RTTC) Monitoring.
The process of tool wear is a complex phenomenon since it is affected by tool material properties, work material characteristics, cutting process variables and possibly machine tool parameters (stiffness, etc.). The complexity of this phenomenon does not lend itself to off-line prediction of tool wear and tool life. Consequently, in manufacturing, it is necessary to adopt a very conservative approach to tool replacement, resulting in frequent and possibly unnecessary tool changes in order to maintain product quality. Further, due to the unpredictable nature of tool wear, off-line or on-line dimensional inspection is necessary to assure workpiece accuracy with consequent additions to product costs.
In order to address these drawbacks and in order to realize the unmanned machining systems of the future it becomes imperative to develop a Real-Time Tool-Condition (RTTC) monitor, with the following capabilities.
With the first capability, it will be possible to determine when the tool edge needs to be replaced. Also, the knowledge of the overall tool condition, in real time, constitutes the last link to the exciting possibility of real time optimization of the machining process. The second capability will enable tool position feedback to improve dimensional accuracy control of the machined component.
The RTTC monitor of the present invention is designed to be capable of these functions in a single point-turning operation.
A review of existing literature indicates that no accurate, commercially viable system for direct monitoring of tool conditions exists. Primitive systems that monitor power or force levels or measure workpiece dimensions are available. These systems have limited use or limited capabilities and do not constitute a satisfactory solution to the problem.
The effort to develop an RTTC monitor with these capabilities was planned to be accomplished in two phases.
The first effort resulted in the identification of a Wear Index (WI) uniquely correlated to tool wear. The second established the validity of the WI and demonstrated the feasibility, applicability and accuracy of an RTTC monitor based on this WI.